8.6 Environmental issues
Carbon dioxide storage in geological formations is a safe way to achieve large-scale reductions in emissions. The dominant safety concern about geological storage is potential leaks that can cause potential local and regional environmental hazards. Leaks can either be slow or rapid. Gradual and dispersed leaks will have very different effects than episodic and isolated ones. The most frightening scenario would be a large, sudden, catastrophic leak. This kind of leak could be caused by a well blowout or reactivation of earlier unidentified geological structures due to for instance microseismic or earth quack events. The most noteworthy natural example of a catastrophic CO2 release was in the deep tropical Lake Nyos in Cameroon in 1986 in which a huge released CO2 gas cloud killed 1,700 people in a nearby village. A sudden leak also could result from a slow leak if the CO2 is temporarily confined in the near-surface environment and then abruptly released.
CO2 being a nontoxic at low concentrations can cause asphyxiation primarily by displacing oxygen at high concentrations. For large-scale operational CO2 storage projects, assuming that sites are well selected, designed, operated and appropriately monitored, the balance of available evidence suggests that it is very likely the fraction of stored CO2 retained is more than 99% over the first 1000 years, implying very negligible risks. However, should leaks occur, the possible local and regional environmental hazards are described in Section 6.4.
At Sleipner CO2 storage project it is important to demonstrate through monitoring and verification procedures to detect potential leaks if any. Monitoring technology that can measure CO2 concentrations in and around a storage location to verify effective containment of the gas needs to be placed. Leakage from a naturally occurring underground reservoir of CO2 such as in Lake Nyos in Cameroon and in Mammoth Mountain, California, provides some perspective on the potential environmental effects. The leaking led to the death of plants, soil acidification, increased mobility of heavy metals and human fatality. These sites are a useful natural analog for understanding potential leakage risks, but for instance Mammoth Mountain is situated in a seismically active area, unlike the sedimentary basins where engineered CO2 storage would take place. Still, we should be wary of undue optimism and continue to question the safety of artificial underground CO2 storage. Given potential risks and uncertainties, the implementation of effective measurement, monitoring, and verification tools and procedures will play a critical role in managing the potential leakage
risks. Continued research on the mobility of the injected CO2 (and the risks associated with its leakage) should be high priorities. Risks associated with leakage from geologic reservoirs beneath the ocean floor are less than risks of leakage from reservoirs under land, because in the event of leakage, the dissipating CO2 would diffuse into the ocean rather than re-entering the atmosphere. But then hazards to ecosystems will be of concern (Section 6.4.3).